Lubricants are extensively utilized in industry and in the automobile sectors for lubricating machineries and materials. A wide range of lubricant base oils is available in the market, which are derived from mineral oil, synthetic oil, refined oil, and vegetable oil. Among them, lubricants derived from mineral oil are most commonly used although they are non-biodegradable and toxic in nature [1]. Extensive use of petroleum based lubricants is creating several environmental issues, such as surface water and ground water contamination, air pollution, soil contamination, and agricultural product and food contamination [2]. Public awareness has resulted in strict government regulations for petroleum based lubricants and hence, new technologies have been aimed at developing lubricant base oils from renewable sources. Synthetic lubricants, solid lubricants and vegetable oil based lubricants are the alternatives to petroleum based lubricants, and they are currently being explored by the scientists and tribologists [3].
Vegetable oil based lubricants are a highly attractive substitute to petroleum based lubricants because these can be environmentally friendly, renewable, non-toxic and completely biodegradable. Vegetable oil based lubricants are preferred not only because of renewability, but also because of their excellent lubricating properties such as high viscosity index (i.e., minimum changes in viscosity with temperature), high flash-point, low volatility, good contact lubricity, and good solvent properties for fluid additives [4]. However, vegetable oil based lubricants have some drawbacks such as poor low temperature properties (opacity, precipitation, poor flow ability and/or solidification at relatively moderate temperature), and poor oxidative and thermal stability (due to the presence of unsaturation) [5]. However, the low temperature properties of vegetable oil based lubricants can be attenuated with the use of additives [4,6]. The oxidative stability of vegetable oil based lubricants can be improved by selective hydrogenation of polyunsaturated C═C bonds of triglycerides [7], or conversion of C═C double bonds of triglycerides to oxirane rings via epoxidation [8-9]. A wide range of reactions can be carried out under moderate reaction conditions by modification of C═C double bonds of triglycerides to oxirane rings [10] and hence, this has received more attention as compared to hydrogenation of C═C double bonds.
Obtaining lubricants from vegetable oils involves three steps: (i) epoxidation of triglycerides to produce epoxy-triglycerides, (ii) ring opening of epoxy-triglycerides, and (iii) esterification. Epoxidized triglycerides are produced industrially by an in situ epoxidation process, in which acetic or formic acid reacts with hydrogen peroxide in the presence of a mineral acid such as sulfuric or phosphoric acid [11]. However, use of a strong mineral acid leads to many side reactions, such as oxirane ring opening to diol, hydroxyesters, dimer formation, and also hydrolysis of oil. Enzymes, resins and heterogeneous catalysts are being used for the epoxidation of oil to overcome the problems connected with the use of mineral acids [12-14].
Goud et al. (2006) reported epoxidation of Mahua oil (Madhumica indica) by using mineral acid (nitric acid and sulfuric acid) as catalyst, hydrogen peroxide as oxygen donor and acetic acid as an active oxygen carrier [15]. Dinda et al. (2008) studied the kinetics of epoxidation of cotton seed oil by peroxyacetic acid generated in situ from hydrogen peroxide and glacial acetic acid in the presence of a mineral acid [16]. Lu et al. (2010) reported the epoxidation of soyabean methyl ester by using Candida Antarctica lipase immobilized on polyacrylic resin in the presence of hydrogen peroxide and free fatty acid [17]. Olellana-Coca et al. (2007) synthesized alkylstearates by using immobilized lipase (Candida Antarctica lipase) followed by epoxidation of oleic acid [13]. Most enzymes were deactivated during epoxidation due to the presence of hydrogen peroxide. Tornvall et al. (2007) studied the stability of Candida Antarctica lipase B during the chemo-enzymatic epoxidation of fatty acids, and reported that temperature control and careful dosage of hydrogen peroxide is essential for chemo-enzymatic processes [18]. Meshram et al. (2011) used the acidic cation exchange resin Amberlite IR-122 for epoxidation of wild safflower oil by using hydrogen peroxide and acetic acid [19]. Mungroo et al. (2008) used Amberlite IR-120H resin for epoxidation of canola oil using hydrogen peroxide and acetic acid/formic acid, and concluded that acetic acid is a better oxygen carrier as compared to formic acid [20]. Sinadinovic-Fiser et al. (2001) studied the kinetics of epoxidation of soyabean oil in the presence of an ion exchange resin, and kinetic parameters were estimated by fitting experimental data using Marquardt method [8].
Limited literature is available on ring opening of epoxy-triglycerides of vegetable oils (also referred to herein as vegetable epoxy-triglycerides) to produce an esterified product. Hwang and Erhan (2001) studied a sulfuric acid catalyzed epoxy ring-opening reaction of epoxidized soybean oil with various linear and branched alcohols followed by esterifying the resulting hydroxyl group with an acid anhydride [6]. Adhvaryu et al. (2005) prepared dihydroxylated soyabean oil by using perchloric acid, and further esterified with acetic, butyric, hexanoic anhydride in the presence of an equimolar quantity of pyridine [1]. Salimon et al. (2010) reported three step processes: epoxidation of ricinoleic acid by using hydrogen peroxide and formic acid, followed by ring opening with various fatty acids by using p-toluenesulfonic acid, and finally esterification with 1-octanol using sulfuric acid [21].